With the increased fuel economy in vehicles in recent years, there has been a strong demand for a pneumatic tire with reduced rolling resistance. Conventionally, various methods for reducing the rolling resistance of a pneumatic tire have been proposed dealing with structural aspects and material aspects, such as reducing the weight of the tire. However, all of these proposals have both merits and demerits and none have realized an absolutely satisfactory reduction in the rolling resistance.
In one aspect of the conventional technology, forming the side wall portions from a foam rubber layer has been proposed. However, when the side wall portions are formed from a foam rubber layer, there is a problem in that cracking resistance is negatively affected. Therefore, Patent Document 1 describes suppressing the negative effect on cracking resistance by setting a foam occupancy area ratio in the foam rubber layer to be low. However, if the foam occupancy area ratio in the foam rubber layer is lowered, heat generated when the tire is running will be easily transmitted from the foam rubber layer. As a result, there are problems in that the foam rubber layer cannot be used in temperature regions where the tan δ, which is deeply related to the rolling resistance of the tire, is lowered, and the effects of reducing the rolling resistance are insufficient. Patent Document 2 describes using a butyl rubber as a foam rubber layer. However, this is unsatisfactory because there are problems in that not only is the heat buildup in the rubber layer itself great, but adhesion with surrounding rubber is insufficient, and is unsuitable as a side wall layer where flexing is great from the perspective of durability. Thus, neither of these proposals has achieved an absolutely satisfactory reduction in the rolling resistance.